Zinc anode gels of alkaline electrochemical cells are prone to electrochemical corrosion reactions when stored at or above room temperature. The alkaline electrolyte in the anode gel corrodes the zinc anode upon contact, forming oxidized zinc products that decrease the availability of active zinc while simultaneously generating hydrogen gas. The rate of corrosion tends to increase as the electrolyte is made more dilute and as the storage temperature rises and can lead to a significant decrease in cell capacity. Cell discharge performance, on the other hand, can be improved making the electrolyte increasingly diluted. It is thus desirable to suppress gas generation when using diluted alkaline electrolytes for increased performance. The additional water aids in the following cathodic reaction:2MnO2+2H2O+2e31 →2MnOOH+2OH31  (for MnO2 cell)  (1)½O2+H2O+2e31 →2OH31  (for zinc-air cell)
However, lowering the hydroxide concentration in the electrolyte can cause the anode to become over-diluted and depleted in hydroxide ions which are needed to sustain the anodic cell reaction:Zn+4OH−→Zn(OH)42−+2e−
The depletion of hydroxide ions can become prominent during medium and high continuous discharge rates and induce depressed cell performance due to anode failure in these cases. Furthermore, when the electrolyte is saturated with zincate Zn(OH)42− produced in the above reaction, the zincate precipitates to form zinc oxide which, in turn, passivates the zinc anode, thereby lowering cell performance. Conventional zinc powders contain particles having a wide distribution of particle sizes ranging from a few microns to about 1000 microns, with most of the particle size distribution ranging between 25 microns and 500 microns. To achieve proper discharge of such conventional zinc powders, a KOH concentration above 34% is conventionally used. At lower concentrations, insufficient KOH is available to the anode and can lead to passivation. Nevertheless, lower electrolyte concentrations are desired because of lower ionic resistance that brings about higher cell operating voltage.
Additionally, hydrogen gas generated during corrosion reactions can increase the internal cell pressure, cause electrolyte leakage and disrupt cell integrity. The rate at which the hydrogen gas is generated at the anode zinc surface accelerates when the battery is partially discharged, thereby decreasing the battery's resistance to electrolyte leakage. The electrochemical corrosion reactions that lead to hydrogen evolution involve cathodic and anodic sites on the zinc anode surface. Such sites can include surface and bulk metal impurities, surface lattice features, grain boundary features, lattice defects, point defects, and inclusions.
To minimize undesirable corrosion and gassing during storage, it is typical to employ corrosion-resistant zinc alloys and to reduce the extent of impurities in the anode. Additionally, organic surfactants and inorganic corrosion-inhibiting agents are commonly added to zinc anodes. Surfactants act at the anode-electrolyte interface by forming a hydrophobic film that protects the zinc anode surface during storage. The inhibitive efficiency of surfactants to increase the corrosion resistance of zinc depends on their chemical structure, concentration, and their stability in the electrolyte.
Among the surfactants known to be effective at controlling gassing are organic phosphate esters such as the ethylene oxide-adduct type disclosed by Rossler et al. in U.S. Pat. No. 4,195,120, incorporated herein by reference. In U.S. Pat. No. 4,777,100, Chalilpoyil et al. disclosed an anode containing single crystal zinc particles with a surface-active heteropolar ethylene oxide additive including organic phosphate esters. In U.S. Pat. No 5,378,559, Randell et al. disclose a galvanic cell that contains a phosphate ester compound to reduce gassing attributed to the brass anode current collector.
Despite their ability to control gel gassing and cell gassing, organic phosphate ester corrosion-inhibiting surfactants also typically decrease high rate discharge performance in electrochemical cells and can adversely affect intermittent cell discharge performance. It is believed that discharge performance suffers as a result of anode failed caused by a combination of zinc passivation, hydroxide ion depletion, and reduction in hydroxide ion diffusion. Therefore, new approaches are sought for inhibiting corrosion and preventing leakage without simultaneously reducing high rate cell discharge performance. At the same time, it is also of interest to develop new classes of corrosion-inhibiting surfactants for use in gelled anodes of alkaline electrochemical cells. Still further, extension of service life by chemical and physical modifications to the anode without sacrificing improvements in corrosion resistance and electrochemical behavior are also highly sought.